The aims of this research are two: 1) To investigate, by electrophoresis on acrylamide gels, simultaneously a number of genetic loci in populations of Drosophila. These studies will include analysis of various populations of the same species, parallel studies of populations of closely related species, and some selected investigation of more distantly related forms. The purpose of this study is to determine what fraction of the genome displays genetic variability between populations of different rank, how much genetic difference there is between closely related forms whether analyzed from the population level within a species or between closely related species, and whether these differences are qualitative or simply quantitative. 2) To investigate, by fingerprint and amino acid analysis, the products of certain selected loci. This analysis will show in greater detail the kinds of genetic change that typify the genetic differences within and between populations and between species. The purpose of this study is to determine the various frequencies of single amino acid substitution, higher orders or substitution, and substitutions of greater complexity such as duplications, deficiencies or rearrangements. 3) In addition we are employing DNA:DNA hybridizing techniques to study the unique DNA sequence homology between all nine of these species. The ultimate aid of these studies is to define, in absolute rather than speculative terms, the kind and quality of genetic differentiation within and between species.